1. Field of the Invention
The invention relates to a process for producing metal films of improved adhesion on non-conducting substrates, in which a UV-curable varnish is used as binder for noble metal compounds (activators), which additionally contains either fillers or solvents or a combination of fillers and solvents.
2. Description of the Related Art
It is known that in order to achieve strongly adhering metal coatings on electrically non-conducting substrates first an adhesion-promoting varnish is applied to the surface of the substrate. However, the disadvantage of this method of pretreatment, in which, for example, ABS polymers are used, is that before the actual metallisation a moulded article coated with such a varnish film must be treated with a pickling solution in order to roughen the surface (DE-OS (German Published Specification) 1,958,839).
Therefore, it has already been proposed to treat non-metallic substrates without prior pickling directly with an activator solution for electroless metallisation, which solution contains, as adhesion promoter, mixtures of, for example, acrylonitrile/butadiene copolymers and, if desired, phenolic resins; the composition of such mixtures is fairly complicated (U.S. Pat. Nos. 3,305,460; 3,560,257). Nevertheless, the adhesiveness of the metal films produced is not sufficient, and the adhesion-promoting polymers do not fulfil the high demands made on the thermal and electrical properties, for example, of printed circuit boards. This is why such a process has so far not been generally accepted in industry.
It is true that the process according to U.S. Pat. No. 4,368,281 in contrast produces better adhesiveness, but this process requires excessively high amounts of activator, i.e. 5 to 16% by weight, relative to the total amount of adhesion promoter.
In combination with the use of small amounts of activator, good adhesiveness has previously only been achieved by treating substrate surfaces with formulations containing, as essential components, noble metal compounds (activators), binders, fillers and more than 40% by weight, relative to the total amount of adhesion promoter, of solvents. Such processes are described in DE-OS (German Published Specification) 3,627,256, DE-OS (German Published Specification) 3,625,587 and DE-OS (German Published Specification) 3,743,780. The good metal adhesion on these formulations is explained by the fact that upon drying of the adhesion promoter rough, porous surfaces are formed in which the electrolessly deposited metal films can be anchored.